AD-AI72 825 HOLOGRAPHIC PROPERTIES OF DNP-128 PHOTOPOLYNERS(U) DUKE 1/1UNIV DURHAM NC B D GUENTHER ET AL 85 SEP 86ARO-23284 i-PH OAALa-86-C-0885
UNCLASSIFIED F/G 14/5 NL
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HE1 10U 181.251 11111.
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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS 1963-A
11 jgj
Lfl
N TITLE
HOLOGRAPHIC PROPERTIES OF DMP-128 PHOTOPOLYMERS
O TYPE OF REPORT (TECHNICAL, FINAL, ETC.)
FINAL
AUTHOR (S)
B. D. Guenther and William Hay
DATE
9/5/86
U. S. ARMY RESEARCH OFFICE
CONTRACT/ GRANT NUMBER
DAALO3-86-C-0005
INSTITUTION
DUKE UNIVERSITY
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HOLOGRAPHIC PROPERTIES OF DMP-128 PHOTOPOLYMERS
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Final I FROMLJ,,J5/TO j2/3J/8[ September 5, 1986 I 4216. SUPPLEMENTARY NOTATION The view, opinions and/or findings contained in this report are those
* of he authr(j) and sh uld not be const ugd as an Efficial Department of the Army position,TV n 'l r ig _ n r •
tP £ e I n n -_ i n L- .cz -- n n c grp~ n -it -g- r h v - P rhl r im " _r,Jf gf l t e n
"7. COSATICODES 18. SUBJECT TERMS (Continue on revtrse if necessary and identify by block number)FIELD GROUP SUB-GROUP
i 19 ABSTRACT (Continue on reverse if necessary and identify by block number)* The holographic properties of the photopolymer DMP-128 produced by Polaroid werei measured. Holograms with a diffraction efficiency of 90% and index modulation in excessi of 0.06 was achieved. Both surface relief and volume phase modulation were observed toI be present. Wavelengths and angular sensitivity were measured. Handling and processing
properties were discussed.
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cop,HINSPECTED
CONTENTS
Chapter 1. INTRODUCTION ...... ................ 1
Chapter 2. THEORY .... ......... .... .... ... . 3
2.1 Holographic Theory . . . .. 3
2.1.1 Interference ........ ............... 3
2.1.2 Hologram formation ....... ............. 6
2.1.3 Hologram reconstruction ..... ........... 9
2.2 The Material ....... ................. 1S
2.2.1 Material parameters .... ............. 16
2.2.2 Shrinkage of the emulsion ... ........... 19
Chapter 3. EXPERIMENT AND ANALYSIS .... ............ 22
3.1 The Experimental Set-up .... ............. 22
3.2 Film Preparation and Processing ... ........... 23
3.3 Measurements ...... ................ 24
3.3.1 Spectral transmission of the system . . . . ... .. 24
4 l
3.3.2 Diffraction efficiency .... ............ 24
3.4 Data and Analysis ...... ............... 26
3.4.1 Diffraction efficiency .... ............ 26
3.4.2 Angular and wavelength sensitivity . ........ 32
3.4.3 Shrinkage ...... ................ 36
3.4.4 Resolution ...... .. ............... 37
3.4.5 Handling and processing ... ........... 37
V'.
Chapter 4. CONCLUSIONS ....... ................. 39
4 4.1 Suggestions for Further Research ... ........... 39
References .......... ...................... 41
.p.
LIST OF FIGURES
Figure 2-1. Interference pattern of two intersecting plane waves . ..... 5
Figure 2-2. Typical geometry for forming gratings. ..... ........... 6
Figure 2-3. Geometry of plane wave interference in a recording medium. ..... 7
Figure 2-4. The propagation vectors j and & of the reference and signal waves and
their relationship to the grating vector I Their respective obliquity
factors are also shown .......... ... ......... 12
Figure 2-5. q/Iq0 vs. 6-angular and wavelength sensitivity of lossless phase
gratings ......... .................... 14
Figure 2-6. A plot of V vs. E for the ideal recording material--lines of constant V0
are shown. ....... ................... 17
Figure 2-7. A plot of V4 vs. V for the ideal recording material--lines of constant E
are shown. ...... ... ................... 18
Figure 2-8. Geometry of shrinkage. (a) Before. (b) After .. ......... 20
Figure 3-1. Experimental set-up to form diffraction gratings .. ........ 22
..Iii.
n--
Figure 3-2. Spectral transmission for the glass substrate and for the exposed polymer
on the glass substrate ....... ................ 25
Figure 3-3. V'vs.E fore = 0and0 = 30. V =.97..... . ....... 27
Figure 3-4. \ vs. E for0 = 0 = 16. V = 1.0 ... ........... 29o I 0
Figure 3-5. Vvs.E for0 = 0and0 = 9. V =.99, V =.85,V0 R 0
=.32 ......... .................... 30
Figure 3-6. V-vs. Vfor0 - = 16. E - 5.0 ... ........... . 31
Figure 3-7. Photograph of surface relief in hologram with 0 = 0 and 0 = 9.1t 0
Magnified 700 times. ..... ................ 32
Figure 3-8. Angular sensitivity measurements for 0 = 0 and 0 = 30. E = 12.1,it 0 0
18.2 ......... ..................... 33
Figure 3-9. Angular sensitivity measurements for 0 = 0 and 0 = 9. E =Rt 0 0
15.8 ......... ..................... 34
Figure 3-10. Angular sensitivity measurements for 0 = 0 = 16. E =R 0 0
15.8 ......... ..................... 35
iv *
Figure 3-11. Wavelength sensitivity measurement--q vs X..............................36
Chapter 1. INTRODUCTION
aver since Denis Gabor discovered the principles of wavefront reconstruction (or
holography) in 1948, efforts have been made to improve the quality of the reconstructed
holographic image. In the early 60's, Leith and Uptanieks improved the performance of
holography by using separate object and reference beams [1]. The invention of the laser shortly
thereafter spurred increased research activity in holography as more powerful coherent sources
made quality images practicable. As in most developing technologies, a major field of
continuing interest is the search for better materials. This thesis is dedicated to the
characterization of a promising new development in holographic recording materials: Polaroid's
DMP-128 holographic recording system
Several material systems are suiable for the recording of holograms, but there are
significant differences in the performance achieved by each. These differences pervade every
aspect of the material: storage, preparation for exposure, exposure, processing and subsequent
usability. Important factors which must be known before the desired hologram can be made
include film sensitivity (analogous to speed in photographic film), linearity of the response to
light, maximum diffraction efficiency, signal-to-noise ratio, wavelength and angular sensitivity
and resolution of the film. Each of these parameters vary from material to material and can
also vary with the thickness or processing of one material. Some of the presently used materials
include silver halide film, dichromated gelatin, photopolymers, thermoplastics and photoresists
[2]. Each of these has its advantages and disadvantages. Dichromated gelatin, the present
industry standard, is capable of very high diffraction efficiency, but great care must be exercised
in processing and developing the hologram. DMP-128 photopolymer exhibits some very
desirable qualities, including long shelf storage time, high efficiency and ease of processing.
Chapter 2 of this thesis will include a review of the principles of holography and theory of
characterization of holograms. The experimental set-up and scope of the experiments along withr1
*' -1-
S Si
ii the results and analysis comprise Chapter 3. Conclusions and suggestions for further research
are reviewed in Chapter 4.
S.
-- 2-
""p. " ;
Chapter 2. THEORY
Holography is the science (and art) of recording the phase and intensity information in a
light signal by means of interfering the signal with another, coherent beam and storing the
resulting interference pattern (a concept very similar to that of modulating an electrical carrier
frequency with a signal frequency). While this work is concerned mainly with one material used
to record this information--Polaroid's DMP-128, a brief review of the principles of holography is
in order before proceeding. This review will be constrained to signal and reference beams
consisting of coherent plane or spherical waves. A description of the DMP-128 holographic
recording system will follow, along with an enumeration of material concerns and parameters.
2.1 Holographic Theory
* 2.1.1 Interference
For a monochromatic wave of frequency f, the electric field V" can be expressed as
- = icos(2wft + 4o), (2.1)
where i is the amplitude of oscillation and 4 is the phase [3]. The phase 40 contains the spatial
dependence of the wave. When two or more sinusoidal waves are summed, the resulting wave
is also sinusoidal, yielding
i cos(2uft + +I) + i cos(2wrft + *2) + . icos(21rft + 4), (2.2)1 1 22
or in complex notation,
Re[iexp[i(2irft + 6) )] + Re[i2exp[i(2rft + +)A]] + (2.3)
= Re[iexp[i(2rft + 40)1].
The latter form is advantageous for most mathematical manipulation and shall be used here,
* .3-
-
N adopting the common convention of dropping the "Re[ ]" while still assuming its existence. As
the frequency will always be held constant in this work and relative, not absolute, phase is the
'* important factor, the temporal phase term exp(i2irft) will be dropped, leaving
iexp(i*4I) + i2exp(i4 2) + .. = iexp(i4) = a, (2.4)
where i is the complex vector amplitude. Complex quantities will be denoted by bold-face type.5,.,
The intensity I of a wave is defined as the time average of the energy flow per unit time
per unit cross-sectional area,
I = IJ2s<' >, (2.5)P
where s is the speed of light in the medium and E is the dielectric constant. In holography, the-4."
intensity I is alternately defined as
I = 2< - >, (2.6)(2.6
which is the square of the amplitude of the wave, or
a ai'i~i. (2.7)
For the purposes of holography, we are generally concerned with only two waves--a reference
and an object wave. In this case, Eq. 2.7 becomes
= ii = i I + i I + 2'i cos(4 -4 ) (2.8)1 1 22 12 2 1
- I + I + 2(ii2COS(OO2 -(0)).1 2 1
In our experimental set-up, both waves are polarized parallel to one another; the dot product
reduces to scalar multiplication of the amplitudes in this case and shall be written in that form.
-4-
% %
In the special case of forming diffraction gratings, both of these waves are plane waves.
Wave normal wove normal*no. I no. 2
F?. 8e 8 F
.,F2 F 0F N F 1
JRgur 2-1. Interference pattern of two intersecting plane waves.
This is presented graphically in figure 2-1. The wavefront crests, separated by the wavelength
X., are represented by F and F for wave number 1 and wave number 2, respectively. Where
11
thesn two wavefronts intersect, they interfere constructively, resulting in planes of the highest
intensity, depicted by the close spacing of the vertical lines, which bisect the incoming wave
normals. These planes lie perpendicular to the plane of the paper. It is this varianc in
intensity which is recorded by the hologram. The relative intensity of the two beams is known
as the K-ratio, where
%K-I I . (2.9)1 2
The importance of K can be seen by considering the maximum and minimum intensities of the
44
*..,.S. -1-. 5 ..,~emc -- r'f w nen cn laew vs
interference pattern. From Eq. 2.8 it cani be seen that when K -1 (corresponding to 1I = 1)2
the intensity of the fringes varies from zero to 41. For K s' 1 (corresponding to I I 12), the
intensity neither falls to zero nor rises to twice that of the total incident intensity. The fringe
visibility V is a measure of this fringe contrast and is defined as
sa" 'min2m
= (2.10)
1 +1I K+ 1
2.1.2 Hologram formad1on
N1
object beam
Figure 2-2. Typical geometry for forming gratings.
A typical set-up for forming diffraction gratings is shown in figure 2-2. The film will be
assumed to be in the z-y plane with the z-axis as the normal to the surface; beams are assumed
to propagate in the y-z plane. The laser is split into two beams which are then spatially filtered
. with a microscope objective and pinhole. Next both beams are collimated and directed onto the
recording medium. The reference and object beams intersect the film plane at angles 8 andRa
*0,1 respectively, as measured from the normal. Depending on the thickness T of the material
and the angles of incidence, the resulting hologram is classified as either a plane (thin) or
volume (thick) hologram. The difference and the consequences will be explained later.
Inside the medium the interfering beams produce planes of periodically varying intensity
Y fK
Figure 2-3. Geometry of plane wave interference in a recording medium.
which extend parallel to the x-axis. The resulting geometry is shown in figure 2-3 [4). Upon
'.9
Ii,
.7.. entering the medium, the reference and object beams are refracted according to Snell's law and
" propagate through the medium at angles 0 and 9 defined by
~ .. . . . . ' . . . . . . •
sine sineR On
= , (2.11)
sine sineI1 0
The resulting amplitude inside the medium is
a = a exp(-i2,r/k(sinO y+cose z)] + a exp[-i2ir'/(sinO Y+cose Z), (2.12)* R.f R R. 0 0 0
and the intensity from Eq. 2.8 becomes
I = I 1+ I + 2(1 R 1) cos[2i/X[(sine - sine )y + (cose - cose )zJJ, (2.13)
where I and I are the intensities of the reference and object beams. At the air-medium
interface, the fringe spacing in the y-direction d isy
d- (2.14)Y
sine - sine0 R
obtained by setting z= 0 and solving for adjacent intensity maxima. The fringe planes bisect R
and 0 and the angle they make with medium boundary, called the slant angle4, is
0 +e
if R 0
- (2.15)
2 2
N- The distance between the crests is called the fringe period d and is equal to
d - d sin,0. (2.16)
The grating vector k is perpendicular to the fringe planes and has the value
K - 2w / d. (2.17)
4-
In order to record the interference pattern described above, the medium must respond in
some way to the intensity of the light. This can be done either by changing the real
transmittance, the thickness, or the index of refraction of the medium [3]. The first type is
known as absorption hologram, while the latter two are known as phase holograms. Also
known as surface relief, the thickness variation changes the phase by controlling the distance the
beam propagates in the medium; the phase is retarded for rays which travel farther in the
material, because the higher index of refraction creates a longer optical path length. In the last
case, the incoming rays are diffracted by the changing index of refraction in the medium. All
three effects may be present in any hologram, but one type usually predominates. DMP-128
changes index of refraction with exposure and records the fringes as a spatial modulation of the
- index of refraction, n = n + n (y,z). In addition to the change of index of refraction, thereo0 m
may be some surface relief involved due to mass transport and changes in density.
2.1.3 Hologram recontrtunilon
A hologram is considered to be a volume hologram if light diffracted by it during
reconstruction passes through several fringe planes [2]. If a hologram is too thin or the fringes
are spread too far apart, the diffracted beam interacts with only one or two fringes and the
hologram is essentially a planar construction. The parameter Q is used to distinguish between
9. volume and plane holograms [41, where
2
* = 2w)L T/nd. (2.18)a
While the transition between volume and plane holograms is somewhat gradual, it is generally
accepted that for 0 2t 10, the holographic properties are based on volume diffraction.
2.1.3.1 Plane holography In the case of a thin hologram, Eq. 2.13 reduces to
(112)
0I + IR + 2(1 I ) cos[2arrA(sinO - sinR )y], (2.19)
.9-
JINL
as the hologram is negligibly thin in the z direction. The recording medium is modulated
proportionately to the intensity I(y), causing a phase shift +(y) on a plane wave propagating
through the developed hologram [1], expressed by
(Iy) a (y) (2.20)
012)= I + I + 2(1 1 cos[2r/)X(sine - sine )y]0 R O R 0 Rt
=4 + 4 1cos[2ir/x(sine ° - sine )y].
The light passing through the developed hologram relative to the light incident on it is called the
transmittance. In the case of a lossless phase hologram, the transmittance is a complex number
of unit value. Thus the transmittance t of a thin hologram is
t = exp(i4 )exp[i4 cos[(2,w/)(sine - sine )y ] . (2.21)
0o 0 it
Neglecting the constant phase factor which has no imaging effect, we can express the
transmittance as a Fourier series
+a
t - i J (4 )exp[(in2iA)(sinO - sine )y], (2.22)al0 Rt
where. J is a Bessel function of the first kind and nwh order. Each of the summed terms
corresponds to a diffracted order of the signal beam [5]. The first diffracted order is
represented by the term for n = 1:
t M (4 )exp[(i2ir/X)(sinO - sine )y], (2.23)
1 1 0 RtI of which the amplitude is simply J For a reference beam of unit amplitude, the maximum
amplitude of the first order Bessel function is .582, thus the maximum diffraction efficiency for
-10-
2 2
thin phase holograms is a = .582 or 33.9%. This relatively low maximum diffraction
efficiency would be severely limiting to practical applications of holography where high
efficiency is required, but as is shown in the next section, higher efficiencies are obtainable in
volume holograms.
2.1.3.2 Volume holography In efficient volume holograms one must consider the attenuation of
the reference beam when analyzing diffraction. Kogelnik adapted coupled wave theory analysis
to holographic volume diffraction gratings in order to account for the high diffraction
efficiencies obtainable [4]. His analysis is followed here.
A wave propagates in the medium according to the wave equation
2 2V a + k a = 0, (2.24)
where k is the propagation constant defined by
k - 2an / k, (2.25)
assuming a lossless medium. The index of refraction is assumed to vary sinusoidally with an
average value n and an index of modulation n, or0 I
n = n + ncos(K 9). (2.26)0 1
where 9 is a unit vector in the y-direction. The incident reference wave a can be written as
a = R(z)exp(-ij-9), (2.27)
where j is the direction of wave propagation. Similarly the amplitude of the diffracted signal
wave a is
as = S(z)exp(-iy9), (2.28)S
%' .fo.
where i is the direction of propagation of the diffracted wave. and I and their relationship to
Y K
1
ft
M -24. The propagation vecton and i of the referene and sigal waves and their relationship to theStating vecor K. 7beir respective obliquity factors are also shown.
t are depicted in figure 2-4. These waves are coupled by a transfer of energy between them.
The amplitude of the total electric field is thus
a - a + a - R(z)exp(-i-t) + S(z)exp(-i& '). (2.29)
If S differs from the Bragg angle 0 by A$ or the reference wavelength differs from the-. ' 0
recording wavelength by AX, the Bragg condition is not met and a "detuning" or "dephasing"
occurs. The dephasing measure 8 is defined as
2
a - As Ksia( ° -)- Ak K /4mrn. (2.30)
The obliquity factors c and c are related to the propagation of the reference and signal wavesIt s
in the z direction, respectively, and are given by
* 12
c - cose (2.31a)
R
c cose - cos4,. (2.31b)S
nd0
They are shown in figure 2-4, using 2wn/k.
When the coupled wave equation is solved for the case of phase transmission gratings-
R(z=0)= 1, S(z=0)= 0-we obtain the amplitude of the emerging signal wave
(L#2) 2 2 (12) 2 2 (1/2)
S(T) -i(c I/c) exp(-it)sin(v + t) /(1 + 6/h ) , (2.32)
where"-" (i/2)
v = wn ITA (c Rc) (2.33)
4 and
B- T/2c. (2.34)
The diffraction efficiency q is defined as the fraction of incident power diffracted into the signal
wave, which is written asa
= (ic/cR )SS. (2.35)
This can now be calculated to be
2 2 2 (1/2) 2 2
-. = sin (v + /(I + t /v (2.36)
In the case when the fringe planes are perpendicular to the medium boundaries (4 = ir/2), this
reduces to:
-13-
2- sin (an T/Xcose0). (2.37)
From Eq. 2.37 it can be seen that the theoretical maximum diffraction efficiency of volume
phase holograms is 100% for an IT/)cos0 = (2x + 1)r/2, for any integer x.
Also of interest are the angular and wavelength sensitivities of a hologram. When AX - 0,
the angular deviation can be expressed as
I - AKdsin(* - e0)t2cs. (2.36)
Similarly, for AS 0, the wavelength deviation can be expressed as
2
- AXK d/8vnC (2.39)
i~S.
0 2
-PIP- 2 .,0v.Ewuaw aent mtv fluspa mip
q. -,
difato efiiny or inrasn vauso ,tesnstvt uv arosadsd oe
0.4
.. 2
0 I 2 3 ,
11g 2-S. q/qo vs. i--ngular and wavelength sensitivity of loslesa phs gratp.
~The sensitivities can be shown as a function of v as in figure 2.5, where 1iI/I 0 is the normalized
diffraction efficiency. For increasing values of w, the sensitivity curve narrows and side lobes
-.14.
become much larger. Increasing v can correspond to increasing index of modulation, thickness
or incident construction angle. For unslanted gratings, the half-power bandwidths can be
approximated by
2A d/T (2.40s). (12)
and
2AX cot(O)d/T. (2.40b). (1i2)
These bandwidth calculations are only approximate as Kogelnik's theory assumes that A8 << I
in order to solve the wave equation (Eq. 2.24) for the coupled signal and reference waves.
2.2 The Material
The Polaroid DMP-128 holographic film is a photopolymer system which produces phase
holograms. Photopolymer systems for holography were first developed in the late 1960's by
Hughes Aircraft [6]. Another notable commercial development of a similar system was by
Booth at Dupont.
A typical photopolymer system consists of a polymer (often called the binder), a monomer
(or monomers) and a dye [6]. The polymer adds physical strength to the emulsion and aids in
the coating of the emulsion substrate. The dye absorbs the incident light, thereby providing the
initiation energy for the photochemical reactions which lead to the polymerization of the
monomer. One photon can provide enough energy to form a molecule chain of hundreds of
monomer units, accounting for the good sensitivity of the DMP-128 material [7]. The
polymerized molecule has a different index of refraction from that of the monomer, resulting in
a gradient in the index of refraction in the material responsible for hologram formation. The
frequency range over which the Polaroid system is sensitive is determined by the absorption
spectrum of the incorporated dye and can thus be altered by the selection of specific dyes.
.,1.
,....~
~.L.~L'~i.N. . * ... . -rn. -4. -
While the exact mechanism of fringe recording in DMP-128 and other photopolymers is not
'- known, several basic tenets are postulated. When light strikes the photopolymer, the monomers
in that region join to form large chains of molecules. This depletion of the monomers in one
region causes a gradient in monomer concentration. The monomers in the higher concentration
regions tend to diffuse to the regions of lower concentration, where they are also polymerized.
This movement of the monomers and polymerization of the material could cause thickness
variations in the material which would yield a surface relief effect. The polymerization into
large molecules is known to cause a change in index of refraction in the bulk material, so this
effect can form volume holograms in which the index of refraction is selectively modulated.
When the material is illuminated with white light after exposure, this thickness variation or index
of refraction modulation is frozen by the polymerization of all the remaining monomer.
Shankoff has theorized that the diffraction in dichromated gelatin is caused by miniscule cracks
in the emulsion which are filled with air [8]. This is also a possibility in DMP-128.
DMP-128 is available in three formulations which contain different dyes: one is sensitive in
s:; the blue region of the spectrum, one in the green and the other in the red. In this report, the
discussion will be limited to the red-sensitive film. The film is also available in varying
thicknesses ranging from three to twenty microns, but this report will be confined to seven
micron emulsions. The emulsion has a bulk index of refraction of 1.53 [91 and is supported on
a flint glass substrate which has an index of refraction that closely matches that of the
- photopolymer.
2.2.1 Material parameters
A holographic recording material has several associated parameters which quantify its
response to the interference pattern which forms the hologram. Among these are sensitivity,
linearity and resolution. These parameters must be considered during hologram formation in
order to achieve the desired results for hologram reconstruction.
% -l%
The sensitivity S is a measure of the amount of change in the material as a function of
exposure energy [3]. It is defined by the equation
SSE EV, (2.41)
, where E is the average exposure over the hologram. For an ideal hologram where the1',2 0
* .~reconstructed wavefront is linearly proportional to the original object wave front, S is
independent of V and E. In real recording materials, however, S is not independent and0
linearity is a non-quantitative measure of how S depends on V and E. A plot of vs. E
'S
0.80V .
0..80.6
,'-
.0.6
... 0.4-
"..
0.2 0.2
o .
0• -,, . 0 2 3 4 5
EO (arbitrary unit)
Ilgu-e 24. A plot of vs. E for the ideal recording material-lines of constant V are shown.0
with various constant values of V is shown in figure 2-6. The characteristics of each value of V
are perfectly straight, indicating linear S. The analogous plot of V vs V for constant E is- 0
shown in figure 2-7.
In an ideal material, all of the incident light would be diffracted into the emerging signal
V7 .17-
.+,r. a 'a *',*.
.~0.6 -
'..
'9.
"
!I o0.6-
'd0.4-
0.2 -
0
0 0.2 0.4 0.6 0.8 1.0V
71guze 2-7. A plot of '4 vs. V for the ideal recording material-lines of constant E are shown.0
beam. Unfortunately, even with perfect holographic technique, the signal is accompanied by
some noise due to imperfections in the hologram and nonlinear recording. The major film
imperfection, neglecting gross macroscopic faults, is that of surface modulation. While the
surface effect can itself form holograms, this causes a generally undesired phase shift in volume
holograms. This effect can be minimized by use of index-matching. Nonlinearity is a more
serious problem. For nonlinear S, the v' vs. E 0 and V curves can be approximated by
polynomial expressions. The first order term accounts for the desired linear response, but the
quadratic and higher order terms cause ghost images and halos. In the case of the diffraction
grating, the higher order terms diffract light into other orders much like a Fresnel zone plate or
*Ronchi ruling.
. Recording resolution is the measure of minimum fringe spacing in the material [3]. Fringe
frequency v is the reciprocal of the period,:--.
; ','.-,,",';':.':':-:.":'':"'","'''"':,'',"":':"":";'' ''-'-"'::'.:" ''':'"" " ""-
v= ld. (2.42)
For transmission holography, the maximum frequency needed can be found by taking most
extreme angles possible in the constructing geometry. Trivially, for the reference and object
beams incident at the same angle, v - 0; for incident beams with the maximum interbeam angle
B = -e = ir/2, v = 2/X . Letting X = .6328 pLm, 0 < v < 3160 cycles/mm. In reflectionRa O a
holography, where the fringe planes are parallel to the medium surface, the fringes are separated
by
XX
d (2.43)1 2 (11) 2 2 (1 )
2(1 - sin 0) 2(n -sin 0)
Letting n = 1.5 and ) =.6328 pm while varying 0 from /2 to 0, we obtain 4700 <S0 a S
(v = lid) < 5700 cycles/mm. If a recording medium can not support as high a resolution as isz a
required, the image will be degraded.
2.2.2 Shrinkage of the emulsion
One difficulty inherent in many holographic media is that of swelling and shrinkage. This
change in size can be quite detrimental to the accurate reconstruction of the recorded image.
The activation and processing steps generally add to or deplete various constituents in the
.. . emulsion. In dichromated gelatin this can actually increase the thickness of the emulsion by
three or four times at certain stages of the processing [2]. Silver halide films tend to shrink by
approximately 15% of pre-exposure thickness as a result of normal development [3]. DMP-128
also exhibits a certain degree of thickness variability as a function of processing.
*1' In general there is little lateral shrinkage in a hologram, as the emulsion is stretched out on
a rigid substrate which prevents this. Excessive lateral shrinkage or swelling would result in the
-19-
5, -,
%. . . .. . . . . . . . . .
oSJ~r
KA,/
(a)
1-gon 21. Geoumtry of -rinkage. (a) Before. (b) After.
tearing or folding of the emulsion and the probable destruction of the hologram. As shown in
figure 2-8, the major effect of shrinkage is the tilting of the fringe planes [10]. The fringe
spacing in the y-direction d remaisconstant due to lack of lateral shrinkage, but the change in7
thickncs results in a change in the slant angle along with the grating period. Cosmquently, the
Bragg angle changes and the hologram does not reconstruct exactly. In effect, the hologram acts
as if it had been formed using different construction angles, symmetric around the new fringe
planes. While the original slant angle was
- ( -) / 2 -a=Can(h / T), (2.44)
the new slant angle +' is
S(O' 0 - o) I 2 - arctan(h T'), (2.45)
Also, retaining the fringe spacing along the y.axis,
- (2.46)
(sine + sie ) (siO' + sine' )
.20.
L M MR..*.
Solved simultaneously, these equations become
sine + sine = sinO' + sine' (2.47)0 it 0 R
and
V' 0 -o 0 ' -0e'o R o R arCtan(h/T)
(2.48)
2 2 arctan(h/sT)
where s = T'/T. For small angles, where sinx tanx = x, Eqs. 2.48 become
(1/3) (80 - 0R) = 8' - 8' (2.49a)0 R 0 It
0 0 =0 ' + 0 ' R (2.49b)
4-21-
Chapter 3. EXPERIMENT AND ANALYSIS
This chapter will describe the experimenal set-up and methods used in making holograms
with DMP-128. The data and results will then be presented and discussed according to the
, ,theory presented in Chapter 2.
3.1 The Eperimental Set-up
Holograms were formed using the Leith-Uptanieks geometry described in Chapter 2 and
~object beam
reference beam
11n3-1. Exumm wt-up to form ddiH d sminp
shown here in figure 3-1. Ile beam from the He-Ne laser (X 632.8 am) is split using a
variable beam-splitter and then spatially-filtered using 10x microscope objectives and 25 ILm
i pinholes. This removes any incoherent elements in the laser light. The beams are collimated
-. and directed onto the film, intersecting the film plane norml at anles of 0 and O for theIts Oa
4,,
nreference and object beams, respectively. Holographic lenses (gratings with focusing power) are
formed by eliminating the collimating lens in the object beam path, leaving a spherically
diverging wave as the object beam.
,:2
During exposure, the film is secured in a film holder adapted from Polaroid consisting of a
prism with a trough secured on the front face. The trough is filled with xylene (index of
refraction = 1.494) which holds the coated substrate against the prism by capillary action. In
order to prevent reflection at interfaces where the index of refraction changes abruptly (which
would cause additional interference in the material and result in a destructive "wood-grain"
pattern), the xylene, the substrate and the prism were all chosen with n = 1.5 in order to match
the index of refraction of the emulsion. The prism is also used instead of a glass plate to
prevent reflection from parallel surfaces back through the emulsion.
The variables of exposure were the angles of incidence, the visibility and the exposure time
(hence exposure energy). Pains were taken to keep all other variables such as activation,
flooding time and chemical processing as constant as possible. The film used during the course
of the experiments came from three separate batches, but is assumed to be uniform in
composition. The thickness of developed samples from two of the batches was measured with a
profilometer and found to be seven microns.
3.2 Film Preparation and Processing
Polaroid's DMP-128 requires activation by absorption of water before exposure and
development [7]. This is achieved in a reproducible manner by exposing the emulsion (which
has been stored in a very low humidity environment) to a controlled-humidity atmosphere for a
prescribed time period. The necessary time interval depends on the ambient humidity and on
the thickness of the material. The humidity is controlled using a saturated salt solution of
. calcium nitrate in a closed container. At equilibrium, the relative humidity above this solution is
51.0%; this remains fairly constant over a moderate temperature range around room
temperature. In order to insure equilibrium conditions inside the chamber, the solution is
constantly stirred with a magnetic bar and the air is circulated in a closed loop above the
solution. For optimum activation under these conditions, the seven micron emulsion is incubated
-23-A .. *J .4
for four minutes. In order to prevent evaporation or absorption of more water into the
emulsion after activation in the chamber is complete, the laboratory air is also maintained at a
relative humidity of 51 - 5% and the xylene, which is hydrophilic, is water-saturated.
After activation, the film is placed in the holder with the emulsion side to the prism and
time is allowed for all vibrations and air currents to cease. After exposure, the film is
illuminated with white light for two minutes to polymerize any remaining monomer and fix the
monomer gradient in the material. The light source is a 60 W tungsten bulb in a reflector held
8" from the film. To insure repeatability in case of any post-exposure monomer diffusion, a
ten-second delay is counted after exposure before the film is illuminated. Next, the film is
placed in the processing fluid for two minutes and then rinsed liberally with isopropanol or
methanol. Afterwards the film is dried in the vapors of boiling isopropanol for one minute.
3.3 Measurements
3.3.1 Spectral transmission of the system
Because the emulsion is coated on a glass substrate, the properties of the substrate must be
taken into account in determining the efficiency of the processed hologram. Also of interest is
the transmission characteristic of the polymer which has been flooded and developed, but which
has not been modulated by an interference pattern. A spectrophotometer plot of the glass
substrate and of the exposed photopolymer on the substrate is shown in figure 3.2. The glass
substrate begins to absorb the light around a wavelength of 360 nm and begins absorbing
strongly at 320 am. The photopolymer begins absorbing at 500 nm and absorbs at wavelengths
lower than 400 nm. This can be considered a baseline value for the scattering of light by the
polymer. In the spectral range used in this work, the absorption can be neglected.
3.3.2 Diffraction efficiency
Al! irradiance measurements were made with a Jodon light intensity meter held
.24.
V*. ,,V '.~ .--. 5 . 5- .. S a . . .
.0 a
0 Q
11tam 3-2. SPeWtra &Maaion for the ga= SubBMW and for to ezpoasd POlYWW eth ls subam.
perenicia to the beam. Measurements of the diffraction efficiency were made by
illuminating a I mm diameter circle in the center of the hologram. In order to account for
Prmne! reflection at the surface of the glass and possible absorption by the polymer, the beam
passing through a portion of the exposed polymer in which no hologram is recorded was
measured and considered to be the incident irradiance; all diffraction efficiency calculations are
made relative to this. In order to measure the angular sensitivity of the hologram, the film plate
was rotated through from 30 to 50 degrees around the constructing reference angle while
-25-
measuring the light diffracted into the first order.
. The wavelength sensitivity measurements* were performed using four different lasers with
different wavelengths. The diffraction efficiency was optimized for the 514.5 mm wavelength
and all other measurements were normalized relative to this value. The other wavelengths used
were 442, 488 and 632.8 nm. All measurements were made at the same incident angle. It must
be borne in mind that this is only an approximate measurement.
Another important concern is the type of mechanism responsible for the diffraction of the
incident beam--an index of refraction gradient or surface relief. If no thickness variation is
found, then the hologram may be assumed to be a volume hologram. A common method to
test for surface relief in holograms is to gate the hologram with a liquid of approximately the
.', same index of refraction; this is known as index-matching. The liquid fills in any thickness
variations and negates any effect of surface relief by eliminating any difference in optical path
length. This was done with xylene and glycerin (n - 1.5). The hologram surface (including
one coated with 150 Angstroms of gold in a vacuum evaporation chamber) was also inspected
with a metallurgical microscope and a scanning electron microscope.
3.4 Data and Analysis
3.4.1 Diffraction efficiency
The diffraction efficiency of a holographic material depends on a number of variables: the
exposure energy, grating period, material thickness and visibility can all be tailored to suit one's
needs. In manufacturing holographic optical elements, maximum diffraction efficiency is
(usually) desired; in making display holograms, linearity is important. The V vs. E and V
plots proposed by Lin [111 provide much of the necessary information.
T o. wavwaq& fmm masurm, wm sraioly palonmd by Tom Sum al ft Univmty of Ra'cu Opms DepuUeM.
.26-
* j s*i*~s ~ -'
q e l
Sqrt (eta)1.0-
0.0+-
* I
I-
0.00Ii
0 17 35 52 70Exposure (mJ/cm^2)
NMSM3-3. Vq' .E fore -O dO = 30. V-.97.
Shown in figure 3-3 is the plot of V vs. E for - 0 and O - 30 for a constant valuef0 R
of V - 0.97. The diffraction efficiency rises quickly with exposure to a maximum around 142
mJI/cm. The efficiency decreases again to roughly two-thirds the maximum efficiency at twice
the exposure. It remains fairly constant thereafter, rising again slightly for very high exposure
q, energies. The initial rise to high efficiency is approximately linear, indicating that the sensitivity
Is not strongly dependent on the exposure in that regime. The diffraction efficiency is high,
reaching a peak of 82.9%. According to Kogelnik's coupled wave theory, the efficiency should
.27-
peak and fall again to zero. This is not observed and there are a couple of possibilities for this.
Kogelnik's theory holds well only for holograms with high values of Q. The Q-factor for this
4 set of holograms is 28.5. While this value is greater than ten, which is quoted by Kogelnik as
the region in which the coupled wave theory holds, the cycling of the efficiency from zero to
one as the index of modulation increases is not always observed to happen unless 0 is very
large [12]. Another possibility is that of surface relief in addition to the volume effects. While
volume and plane holograms are theoretically distinct, the dividing line between them is blurred
and most holograms show some effects of both types. This is reinforced by the observation of
fringes in all of the holograms using the metallurgical microscope.
The plot of V' for 0 R - = 16 shown in figure 3-4 is very similar to the previous plot.at o2
The efficiency reaches a peak of 90.0% for an exposure energy of 6.7 mJ/cm and decreases to
an efficiency of around 50% at twice the exposure. The diffraction efficiency appears to rise to2
a second peak near E = 20 mJ/cm. The index of modulation of the hologram with exposure02
energy of 16.7 mJ/cm can be calculated to be .061.
Figure 3-5 shows a plot of V' vs. E for S = 0 and 0 = 9 and constant values of
visibility. For V = .99, the diffraction efficiency rises quickly to an apparent peak near 162
mJ/cm and then decreases much like the aforementioned examples. In agreement with Lin [10],
the diffraction efficiency for V = .85 rises more slowly than that for V = .99. The slope is
approximately linear, although not completely. Again, the peak efficiency is followed by a drop
and then a slow rise. The diffraction efficiency for V = .32 rises slowly, but quite steadily,2
only reaching a peak of less than 50% for an exposure of 50 mJ/cm before leveling off.
The other plot proposed by Lin [111 is a plot of Vq vs. V for constant E. An example of
this is shown in figure 3-6 for 0 = 0 = 16 and E = 5.0. The plot is almost linear, showing,a 0 0
only small dependence on visibility. For a hologram with a relatively uniform exposure
intensity, one can see the limits of linear recording from this plot. Many holograms, such as
%:4
16. -. , . . .. . - .. .. , •
Sqrt (eta)".; IcZ I I--0 0
0.00 6 12 18 25
Exposure (mJ/cm^2)p.,
lrm, 34. Vm.s .E fore - e - 16. V-1.o.0 R 0
diffusely-lit object holograms are fairly uniform in exposure, but not in visibility. For
holographic optical elements such as diffraction gratings, the visibility is constant and the plots
of V~ vs. E are more revealing.. 0
Several approaches were tried to resolve the question of the mechanism of diffraction.
. Index-matching produced inconclusive results. When index-matching using xylene, the
diffraction efficiency decreased from 61.2% to 1.7% on one hologram made with a total
-29-
" "- .
Sqrt (eta)10 I I I I I
-- A A
0 0
o0, oI ° I I I
-0 40 80 i20~Exposure (md/cm'2)
Har.4.V ,.Eto, ,,o-,e -9. v -.9,i.5,v -.32
0 R o
interbeam angle of 30 degrees (d - .80 Lm). This can not be due completely to negating any
surface relief as the maximum diffraction efficiency of such holograms is 33.9%. The hologram
also disappears when wetted with methanol or isopropanol and returns to the original diffraction
efficiency after drying. These liquids (with low surface tensions) may be filling in tiny cracks or
voids in the polymer which account for the change in index of refraction. This method of/hologram formation in phase gratings was proposed by Curran and Shankoff [81 for dichromated
gelatin and could also be present in the photopolymer. Index-matching was also performed
.30.
III I I
0.8-
0.7
0.6
0.5-
0.4-
0.3-
0.2-
"" 05
0.1--
0.00.0 0.2 0.4 0.6 0.8 1.0
Visibility
PIPM.3-C V"vs. V for 0 0 16. E " 5.0.It o 0
using glycerin (n - 1.5), a more viscous liquid. The observed diffraction efficiency of the same
. hologram decreased to 6.6% upon application of the glycerin. Assuming that the glycerine does
not penetrate the hologram as easily as do the alcohols and xylene, the smaller drop in
diffraction efficiency could be attributed to not filling in all or as many of the voids. Shankoff
noted that in dichromated gelatin the emulsion had pulled away from the substrate, creating
voids between the substrate and the emulsion. This could account for the higher diffraction
efficiency when gated with glycerin rather than zylene-having a higher surface tension, the
-31-
4-
M90m 3-7. Photo aph of suASM relief il hologrm with S - 0 1m S -0 9. Mapnified 700 dMes.a O
glycerin could not flow as well as the xylene and fill deep voids. A second hologram, with a
total interbeam angle of 9 degrees, originally had a diffraction efficiency of 71%, but fell to
12% when gated with glycerin and 4.5% when gated with xylene. The holograms were also
i. observed under a metallurgical microscope for possible thickness variations. Surface modulation
was visible in all the holograms, even the holograms with smaller periods. The relief was not
measurable, but appears to be less than one micron. Small scale surface roughness was also
visible in all holograms. A photograph of a hologram which was coated with a 15 nm layer of
-~gold in a vacuum evaporator is shown in figure 3-7. The period was measured to be 2.4 pm
which corresponds to the calculated value for the incident angles of 8 = 0 and 0 - 9. TheR 0
same hologram was viewed in a scanning electron microscope. The electron beam was visibly
destructive to the hologram and no surface relief could be seen, although surface roughness and
irregularities were visible at magnifications from 500 to 20000 times.
4. 3.4.2 Angular and wavelength sensitivity
Angular sensitivity measurements were made on four sets of slides with different incident
angles: O -0 and -0 9, O R 0and0 - 12, S W 0and S 0 30 andS -0 - 16.
-32-
% .jx?,
,," 1
-'.
Etae1. 0-
,-F- 2o
- -20 0 10Reconstr. Angle
Fismt 34. Angua eitivity meauremnt for 6 0 and 8 30. E =12.1, 18.2.
modulation, the side lobes are seen to increase as predicted by Kogelnik (4]. The half-power
bandwidth is 8 degrees for both; it should show no change for increasing efficiency and does
not. The calculated half-power bandwidth from Eq. 2.38a is 6.5 degrees. For the plot shown
4 in figure 3-9, the calculated half-power bandwidth is 19.6 degrees; when measured, it is 22
degrees. From these two plots, it can easily be seen that the angular sensitivity becomes much
*%' -, .. .
smle fo raeRnlso ni ect . nl
.33o
":'-"~~~~~~~~ ~~~~~ %hw ifgr - r w esrmnsfre n 0 o nraigidxo
Eta1.0' iiiiiliiiiiiiiiii,1 1 iiiii iIII I llII II I
0.0 III II
:--20 -i0 0 10 20SReconstr. Angle
- ': lllInrm 39. Angular wmitii4 nwmur==ft for 0 . -= 0 and 0 - 9. E -= 15.9.
-- 1 0
A progression of plots shown in figure 3-10 presents quite clearly the increasing efficiency
A of the side lobes as the index of modulation increases. The exposure energies vary from 3.3 to
- 223.4 mJ/cm, while the index of modulation varies from .02 to greater than .06. While the
efficiency of the center lobe increases and decreases as expected, though to a lesser degree than
Kogelnik's theory predicts, the half-power bandwidth remains the same. The calculated value is
6.1 degrees and the measured value is approximately 8 degrees.
.34.
-p." %
Eta1.0 l
to. 0
-' 1/I
0 05 15 25 35 45
Reconstr. Angle
g., m 3m0. Anguar mitvity mn turmes for . = -0 16. E 0 iS.8.a . o o
- The wavelength sensitivity was measured for a hologram with incident angles of
. - 0 - 16. The diffraction efficiency was peaked at 21.2 degrees for the green 514.5 nma o
light and then was measured for that same angle with wavelengths of 442.0, 488.0 and 632.8
nim. The result is shown in figure 3-11. The half-power bandwidth is measured to be
approximately 180 nm; the calculated value is 142 nm. The experiment itself was not very exact
and Kogelnik's theory is also approximate, so the correlation is acceptable.
.33-* .... , ..
Eta
i.-
S0.0 111111 lili,l1111111111l11 111111IIIIIII1 I
440.0 490.0 540.0 590.0 640.0
Wavelength (nm)
Rpgm 3-U. WavW=X&t =Mitvity mmimmmt-q vsX
3.4.3 ShrinkageJ
At different stages in processing, the thickness of DMP-128 changes. During activation the
emulsion absorbs water and increases in thickness. As the exposure is made with the emulsion
in this state, the thickness T at this point is the thickness for hologram formation. The
measured thickness after processing is seven microns. Not being able to measure the thickness
during exposure, it must be calculated from the theory in Chapter 2. Solving Eqs. 2.48a & b
.36-
for an angle of 9 degrees, the shrinkage factor s is seen to be .82. This translates to a
shrinkage of about 18%. As a rough confirmation of this shrinkage factor, reflection holograms
recorded in the red "play back" in the green-blue region. In reflection holograms the fringe
planes are parallel to the substrate, so the period changes directly with the thickness. A
spectrophotometer measurement of the absorption by an in-line reflection hologram constructed
with the 632.8 m line of a He-Ne laser showed a peak absorption around 480 nm, so a rough
estimate of the shrinkage factor is s = 480/632.8 = .76. Since the object signal was not a plane
wave in this case, this result must be regarded as a rough estimate only.
3.4.4 Resolution
The resolution requirements for phase transmission holograms constructed with a wavelength
of 632.8 nm range from 0 cycles/mm (for an inter-beam angle of zero degrees) to 3160
cycles/mm (for an inter-beam angle of 180 degrees). The range of resolutions used in this study
extended only from 410.4 to 1335.6 cycles/mm. These spatial frequencies were well within the
resolution capabilities of DMP-128.
3.4.5 Handling and processing
DMP-128 is an extremely easy film to use. The film can be stored for up to a year at a
relative humidity less than about 35%. Some of the film used in this experiment was 6 months
old at the time of exposure; a maximum diffraction efficiency of 90% was achieved with one of
these plates. With the exception of silver halide, most other films have a shelf life of days or
hours, not months, once they are formulated or coated on a substrate. After flooding with light,
DMP-128 can be developed and fixed with one chemical bath and then dried. Some films, one
of which is the Dupont photopolymer, can develop themselves with no processing and thus
produce near real-time holograms, but they require fixing in order to prevent the hologram from
degrading in a matter of hours or days. DMP-128 must be fixed and developed chemically, but
the whole process requires only five to six minutes and the holograms show little sign of
.37-
• '- " . " "'' """- "" - ,' -' 'W .' " " "- " " . .. . . .. ' " , ,- , ", * ,1 Z
degrading, even under conditions of high humidity. Another advantage of DMP-128 is the
-option of using safelights during all stages of exposure and processing. In the case of red-
.' Z-, sensitized film, the laboratory may be illuminated with blue safe-lights even after the emulsion is
activated.
The activation of the film in a controlled humidity chamber must be determined
experimentally, but once the optimal activation time has been found the incubation period need
only be held constant; the film can be very activated in an easily reproducible manner. In order
to prevent the loss or additional absorption of water after activation, the laboratory must be
maintained at a relative humidity near 51%. The shrinkage discussed above is a disadvantage,
but with additional processing baths, this can be at least partially corrected. DMP-128 has been
observed to crack along the fringe planes when overexposed or subjected to repeated drying.
This is a result of stressing the material too much and can generally be avoided.
.13
V..
.36
Chapter 4. CONCLUSIONS
In this thesis a new holographic recording system was tested. Polaroid's DMP-128 film is a
photopolymer medium which produces phase holograms. The phase change is caused by the
polymerization of monomers by photo-initiation. A hologram with a diffraction efficiency of
90% was produced. An index of modulation in excess of .06 was achieved for high exposure
energy without harming the integrity of the emulsion. Regions of linearity for V plotted
against visibility and exposure were shown. The mechanisms responsible for diffraction by the
photopolymer were investigated; both surface relief and volume phase modulation are observed
to be present. Wavelength and angular sensitivity were measured and found to agree closely
with Kogelnik's coupled wave theory. Resolution and the extreme ease of handling and
processing of DMP-128 werealso discussed.
4.1 Sugesadons for Further Research
All of the holograms fabricated in the process of this research were formed with X. = 632.8
nm. The emulsion, however, is sensitive over a broader range of wavelengths than this. Others
have obtained an estimate for the spectral sensitivity of DMP-128 [12], but a rigorous experiment
has yet to be conducted to verify this. To accomplish this, holograms should be constructed at a
given intensity for a range of wavelengths and then be measured for diffraction efficiency.
Additional investigation could be directed toward a similar characterization of the blue- and
green-sensitized formulations of DMP-128. The more complicated problem of reflection
holography can also be explored much like transmission has been in this work.
The major thrust of any additional work should be centered around the processing of the
film. Variables such as effects of flooding time and delay time before flooding need to be
measured to check for diffusion of the monomer. The intensity and duration of the flood also
merits investigation. The chemical processing could be tested. Variations in the chemical
-39-
* .W, V ~ %
components should be tried and their effects noted. Closely linked with this would be an
-, investigation into methods to swell the emulsion back to its thickness during exposure.
40,
----
References
[11 Smith, H. M., Principles of Holography, 2nd ed., John Wiley & Sons, New York, 1975.
[21 Smith, H. M., ed., Holographic Recording Materials, Springer-Verlag, Berlin, 1977.
[3] Collier, Robert J., et al., Optical Holography, Academic Press, Inc., Orlando, 1971.
(4] Kogelnik, H., "Coupled Wave Theory for Thick Hologram Gratings", Bell System
Technical Journal, 48:9, Nov. 1969, pp. 2909-2947.
[5] Guenther. B. D., to be published.
[61 Rick Feinberg, "An Interview with Richard T. Ingwal: Photopolymers for Holography."
Optical Engineering Reports, No. 31, July 1986, pp. 1, 4.
(7] R.T. Ingwall, A. Stuck, W.T. Vetterling, "Diffraction Properties of Holograms Recorded
in DMP-128", Polaroid technical report no. 615-05.
[8] Curran, R. K. and Shankoff, T. A., "The Mechanism of Hologram Formation in
Dichromated Gelatin", Applied Optics 9, No. 7, July 1970, pp. 1651-7.
[91 Ingwall, R. T. and Fielding, H. L., "Hologram Recording with a New Polaroid
Photopolymer Recording System", SPIE Vol. 523 Applications of Holography (1985), pp.
306-12.
[101 Vilkomerson, D. H. R. and D. Bostwick, D., "Some Effects of Emulsion Shrinkage on a
Hologram's Image Space", Applied Optics 6, No. 7, July 1967, pp. 1270-1.
-41.
.. p* , ~ . pr
[111 Lin, L. H., "Method of Characterizing Hologram-Recording Materials", Journal of the
-Optical Society of America, Vol. 61, No. 2, February 1971, pp. 203-8.
.
[121 Stone, Thomas, personal communication.
[13] Norton, M. C., "Holographic Applications of Polaroid DMP-128", no published date.
- 42-
-42-
Iv~n"ba
pI
V III UIL
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